Click on Output Config under Channel Configuration in the Audio Preferences.

Activate all outputs as shown in the screenshot below.

Note: If you know exactly which physical input / output channels you are going to use, you may leave the unused channels deselected.

Close the Preferences and go to Live's Session View.

Open the I-O settings by clicking the corresponding button on the right edge of Live's Session View.

In the Master track, select an output pair for your Master Out and another output pair for your Cue Out (prelisten). In this example we are selecting channel A (1/2) as our master output and channel B (3/4) as our monitor output.

Ableton Live's master output will now play through the output channel A (1 and 2) of the KOMPLETE AUDIO 6, which you can connect to your speaker system. You can use any of Ableton Live's audio routing options to route audio from and to the the inputs and outputs of your Native Instruments audio interface, including the I/Os of individual Audio and MIDI Tracks as well as the External Audio Effect and External Instrument plug-ins. Please refer to the Ableton Live documentation for more information.